Method for producing semicrystalline polylactic acid articles

ABSTRACT

Amorphous sheets of PLA resins are thermoformed by heating the sheets until they become semicrystalline, and then forming the sheets on a relatively cold mold. Semicystalline formed articles having improved heat resistance are made by the process.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to methods for formingheat-resistant, semicrystalline articles from polylactic acid.

[0002] Polylactic acid (PLA) is useful for making films, fibers, andvarious types of formed articles. One limitation on its use in some foodpackaging and other applications is its tendency to deform when heated.For example, many food packaging applications require the resin to besubjected to the temperature of boiling water without significantdeformation. PLA articles often cannot withstand such temperatures.

[0003] PLA tends to exist in an amorphous state when formed into thesekinds of articles. Experience with a more conventional resin,poly(ethylene terephthalate) (PET) has shown that better resistance toheat can be obtained if the polymer has a greater amount ofcrystallinity. For PET, this has resulted in a specialized thermoformingprocess for inducing crystallinity into PET articles. In cPETthermoforming, a PET sheet is heated until soft enough to be formed,then transferred to a hot mold and formed under vacuum and pressure. Thetemperature and residence time in the mold are such that crystallitesform in the resin. Once the needed crystallinity is obtained, thearticle is transferred into another mold of identical dimensions. Thissecond mold is held below the glass transition temperature (T_(g)) ofthe PET resin, usually near room temperature. The colder temperatures“quench” the resin, “locking in” the as-formed dimensions. If theresulting PET resin is sufficiently crystalline, it may withstand usetemperatures some 20-150 C. higher or more than amorphous PET.

[0004] The PET thermoforming process has the drawbacks of requiring twomolds, which increases capital investment and operating costs, and longforming times in the mold to allow crystallization to be completed,which reduces output per unit time and thus increases costs.

[0005] As mentioned, the cPET thermoforming process is amaterial-specific one that is designed around the particularcharacteristics of PET resin.

[0006] It has been recognized that PLA, like PET, can be formed into amore crystalline state by subjecting it to certain temperatures. SeeKolstad, “Crystallization Kinetics of Poly(L-lactide-co-meso-lactide)”,J. Applied Polymer Science 62, 1079-1091 (1996). As described byKolstad, the rate of crystallization is affected by various factors,including the lactic acid enantiomer ratio, the use of nucleatingagents, and the thermal history of the polymer (i.e. time atcrystallization temperature and/or cooling rates).

[0007] Nonetheless, no cost-effective commercial process has beendeveloped for making formed crystalline PLA articles. It would bedesirable to provide such a process, as it would permit PLA to be usedin end-use applications that require improved heat resistance.

SUMMARY OF THE INVENTION

[0008] In one aspect, this invention is a method for making formedheat-resistant PLA articles, comprising heating a sheet of an amorphous,crystallizable PLA resin until the sheet has obtained a surfacetemperature from about 80 to about 155° C., then thermoforming theheated sheet on a mold that is at a temperature below 80° C.

[0009] In a second aspect, this invention is a method for making formedheat-resistant PLA articles, comprising heating a sheet of asemicrystalline PLA resin having a crystallinity of at least 10Joules/gram to a temperature at which the sheet can be thermoformed butat which the crystallinity of the sheet is not reduced below 10Joules/gram, and then thermoforming the sheet on a mold that is at atemperature below 80° C. to produce a formed article of PLA resin havinga crystallinity of at least 10 Joules/gram.

[0010] In another aspect, this invention is a method for making formedheat-resistant PLA articles, comprising heating a sheet of an amorphous,crystallizable PLA resin to a temperature at which the PLA resin formscrystallites and for a time sufficient to impart a crystallinity of atleast 10 Joules/gram to the PLA resin, and then thermoforming the heatedsheet on a mold that is at a temperature below 80° C. to produce aformed article of PLA resin with a crystallinity of at least 10Joules/gram.

[0011] This invention provides a simple, efficient method for makingformed articles with improved heat resistance from crystallizable PLAresin. Cycle times as short as about 3 seconds or even less for theforming step can be achieved with this method using commercial-scaleequipment. Because only a single mold is necessary, capital expense andoperating expenses are minimized. The crystallinity of the PLA resin canbe controlled over a wide range by adjusting time and temperature duringthe heating step, and through control of the composition of the PLAresin itself, as described more below. As certain properties of the PLAresin (notably heat resistance) vary with the crystallinity, thisprocess provides a method whereby easy control over those properties canbe obtained. In particular, this invention provides a rapid andinexpensive process for making formed PLA articles that can resistexposure to temperatures of 100° C. or more with minimal or nodistortion. This permits the articles to be used in a variety ofapplications, in particular food packaging, in which the article and itscontents are to be heated.

DETAILED DESCRIPTION OF THE INVENTION

[0012] In this invention, a semicrystalline sheet of PLA resin isthermoformed on a mold that is at a temperature of 80° C. or less. Thesemicrystalline sheet of PLA resin is obtained by heating a sheet ofamorphous PLA resin to a specific temperature range until the desiredcrystallinity is obtained. The resulting semicrystalline sheet can beimmediately thermoformed, or else cooled and later reheated to athermoforming temperature under conditions that the crystallinity ismaintained and then thermoformed.

[0013] In the preferred process, the heating step is performed as partof the overall thermoforming process. In the first step of the preferredprocess, a sheet of amorphous but crystallizable PLA resin is heated.The function of the heating is two-fold—to soften the sheet so that itcan be formed in the subsequent step, and to introduce crystallinityinto the PLA resin. The conditions of the heating are selected toachieve these purposes.

[0014] Softening is performed such that the sheet can be thermoformedunder commercially reasonable conditions, without softening the sheet somuch that it is too fluid to transfer to the thermoforming mold and beformed into a part.

[0015] Crystallinity is introduced such that the sheet develops acrystallinity during the heating step of at least 10 Joules/gram,preferably at least 15 Joules/gram, more preferably at least 20Joules/gram, even more preferably at least 24 Joules/gram, to about 55Joules/gram, preferably to about 45 Joules/gram, even more preferably toabout 40 Joules/gram.

[0016] Crystallizable PLA resins are those that will form crystallitesrapidly when a sheet of the resin is heated to a sheet surfacetemperature range which lies above the T_(g) but below its meltingtemperature (T_(m)). The temperature range to which the PLA resin mustbe heated will be a characteristic of the particular PLA resin, and willdepend largely on the lactic acid enantiomer ratio and the presence ofnucleating agents and/or plasticizers, as described more fully below. Ingeneral, however, the temperature at which the requisite crystallizationwill occur within commercially reasonable time frames is from about 80°C. to about 160° C. and is more typically from about 90° C. to about150° C.

[0017] Thus, one method of controlling the process of the invention isto monitor sheet surface temperature. Sheet temperature is generally asuitable process control parameter for sheet thicknesses that aretypically used in thermoforming applications, such as up to about 250mil, preferably about 10-100 mil, even more preferably about 15-50 mil,unless extremely rapid heating rates are used. A thicker sheet, or onethat is heated very rapidly, may exhibit a significant temperaturegradient between the surface and the center. Under these conditions,therefore, surface temperature measurements alone may be less suitableas a process control parameter. For the best results, it is desirable tointroduce crystallinity throughout the thickness of the PLA resin,rather than only near the surfaces of the sheet.

[0018] It is anticipated that in most cases, heating conditions will beestablished empirically with respect to the particular equipment andparticular PLA sheet that is used. These empirically derived heatingconditions may be developed by establishing suitable and/or optimaltemperatures to which a particular PLA resin sheet should be heated, andthen relating those temperatures to particular controllable processingconditions such as heating time, line speeds, heater and/or oventemperatures, power to be supplied to heating apparatus, and the like.Alternatively, heating conditions can be empirically derived bymeasuring the crystallinity of the heated PLA sheet while varyingprocess parameters.

[0019] The heating step can be carried out in any convenient manner,such as convection heating, radiant heating (using devices of varioustypes such as visible light, infrared radiation, microwave radiation,and the like), conductive heating (such as by passing the sheet over aheated surface or between heated surfaces such as heated rollers) andinduction heating. In order to keep cycle times short, heating ispreferably done quickly and uniformly without scorching the sheet orforming significant localized hot spots. The sheet is conveniently heldin a clamping frame or other apparatus to give it physical support andto facilitate transfer in and out of the heater and/or to the subsequentforming step.

[0020] The heating step may also be performed as part of the process ofextruding the sheet. In this variation, the PLA resin is first extrudedinto sheet form. Because extrusion temperatures are generally higherthan the melting temperature (T_(m)), it is necessary to cool the sheetin order to reduce the temperature below T_(m) and into the temperaturerange at which crystallization occurs. The heating step of thisinvention can be accomplished by adjusting the temperature of thefreshly extruded sheet into the aforementioned temperature ranges, andholding the temperature there until the requisite crystallinity has beendeveloped. The crystallized sheet can then be cooled below the T_(g), inorder to be thermoformed in a separate, later step. In such a case, thesheet will be re-heated to the thermoforming temperature (but not to sohigh a temperature that crystallinity is destroyed) when thethermoforming process is performed. Alternatively, the crystallizedsheet may be fed directly into the thermoforming step.

[0021] Following the heating step, the PLA sheet is transferred to amold and thermoformed. The transfer to the mold is performed so that thePLA sheet remains at a temperature suitable for thermoforming until thetransfer is complete and the thermoforming accomplished. The mold is ata temperature below 80° C., preferably below the T_(g) of the PLA sheet,more preferably no greater than 50° C., and especially no greater than35° C. Because of the relatively cold mold temperature, the PLA sheetquickly hardens into the desired shape in the mold. Some additionalstress-induced crystallinity may be introduced due to the orientation ofthe polymer during the thermoforming process, but the amount isgenerally small.

[0022] Thermoforming is accomplished by positioning the softened andcrystallized sheet over a male or female mold, and drawing and/orpressure forming the sheet on the mold to form a molded part. The moldis most typically a female mold. Multiple formed parts can be madesimultaneously or sequentially from a single sheet. Except for the moldtemperature, which is held below the temperatures discussed above, theprocess of this invention can be conducted using conventional types ofthermoforming apparatus, which are adapted if necessary to provide ameans to maintain the mold at the requisite temperature. Examples ofsuch apparatus and general methods are described, for example, by Thronein “Thermoforming Crystallizing Poly(ethylene Terephthalate) (CPET)”,Advances in Polymer Technology, Vol. 8, 131-146 (1988). Drawing isperformed using vacuum, and is the preferred method. The mold mayinclude a male half that is inserted into the female half during theprocess to provide male mold forming. It may also be desirable toprestretch the sheet; if so a pressure cap or other prestretching devicemay be used and actuated prior to drawing the sheet into the mold.

[0023] The thermoforming step is preferably operated such thatthermoforming cycle time (time to complete one thermoforming cycle andget ready to perform the subsequent cycle) is minimized. Thermoformingcycle times are advantageously less than 20 seconds, preferably lessthan 10 seconds, more preferably no more than 5 seconds, and even morepreferably no more than 3 seconds.

[0024] Once the part is formed and cooled below its T_(g), it isdemolded and separated from other parts and trimmed if necessary.Various downstream operations, such as applying graphics or labels,assembly to other parts, packaging and the like can be performed ifneeded, depending on the type of part and its intended use.

[0025] By “PLA sheet”, it is meant a sheet of a thermoplasticpoly(lactic acid) homopolymer or copolymer containing at least 50% byweight (based on the PLA resin) of repeating units derived from lacticacid. The PLA is more preferably a homopolymer of lactic acid. The PLAresin may be blended with small (up to about 50% by weight, based on theweight of the polymers) of another polymer which is not a PLA, but ispreferably not such a blend. The preferred PLA resin is a homopolymer ofeither L-lactic acid or D-lactic acid, a random copolymer of L-lacticacid and D-lactic acid, a block copolymer of L-lactic acid and D-lacticacid, or a mixture of two or more of these. As discussed below, theratio of the lactic acid enantiomers and the manner in which they arecopolymerized (i.e., randomly, block, multiblock, graft and like)greatly influences the ability of the PLA sheet to crystallize in thepresent process.

[0026] The PLA resin can be formed by polymerizing lactic acid or,preferably, by polymerizing lactide. Thus, the term PLA resin is usedherein to include polymers made by polymerizing lactide. Lactide is adimeric form of lactic acid, in which two lactic acid molecules arecondensed to form a cyclic diester. Like lactic acid, lactide similarlyexists in a variety of enantiomeric forms, i.e., “L-lactide”, which is adimer of two L-lactic acid molecules, “D-lactide”, which is a dimer oftwo D-lactic acid molecules and “meso-lactide”, which is a dimer formedfrom one L-lactic acid molecule and one D-lactic acid molecule. Inaddition, 50/50 mixtures of L-lactide and D-lactide that have a meltingtemperature of about 126° C. are often referred to as “D,L-lactide”. Anyof these forms of lactide, or mixtures thereof, can be copolymerized toform a PLA resin for use in this invention. The L/D ratio in the PLAresin is controlled through the ratio of these enantiomeric forms oflactide that are used in the polymerization. In an especially preferredprocess mixtures of L-lactide and meso-lactide are polymerized to form apolymer having a controlled level of D-lactic acid enantiomeric units.Suitable processes for polymerizing lactide to form PLA havingcontrolled L/D ratios are described, for example, in U.S. Pat. Nos.5,142,023 and 5,247,059, both incorporated herein by reference.

[0027] The PLA resin may be compounded with various additives andprocessing aids such as nucleants, other inorganic fillers,plasticizers, reinforcing agents, slip agents, lubricants,UV-stabilizers, thermal stabilizers, flame retardants, foaming agents,antistatic agents, antioxidants, colorants, and the like, with finelydivided inorganic solids being of particular importance as discussedmore below.

[0028] The PLA sheet is amorphous and capable of being crystallized tothe extent of 10 Joules/gram through heating at a temperature betweenits T_(g) and T_(m). Preferably, it is capable of being crystallized toa crystallinity of at least 15 Joules/gram, more preferably at least 20Joules/gram, even more preferably at least 24 Joules/gram, especially atleast 30 Joules/gram. For purposes of this invention, a PLA sheet isconsidered “amorphous” if it exhibits a crystallinity of less than 10Joules/g when measured by differential scanning calorimetry (DSC) asdescribed more fully below.

[0029] The composition of the PLA sheet is also preferably such that thesheet can be crystallized to the desired extent in a short period. Threeimportant parameters affecting this are the ratio of the lactic acidenantiomers in the PLA resin, the use of nucleating agents and the useof plasticizers.

[0030] The ability of PLA to crystallize, all other things being equal,is greatest when only one of the lactic acid enantiomeric forms ispresent in polymerized form in the polymer. Thus, homopolymers ofL-lactic acid or D-lactic acid are the forms of PLA that tend tocrystallize most completely and rapidly. In random copolymers of the L-and D-enantiomers, the ability to crystallize falls off rapidly as moreof the second enantiomer is present. In this invention, it is preferredto use a PLA resin in which the lactic acid enantiomer ratio is at least90:10, preferably at least 95:5, more preferably at least 98:2, to about99.9:0.1, more preferably to about 99.5:0.5, even more preferably 99:1.It is unimportant in terms of performance whether the predominant isomeris the L- or D-form. PLA resins having predominantly the L-isomer aremore readily available commercially, and are preferred for that reason.

[0031] The proportion of “D” and “L” lactic acid repeating units inenantiomeric polylactic acid (PLA) copolymers can be accomplished byhigh performance liquid chromatography. A suitable chromatograph is aWaters LC Module I HPLC with a Sumichiral OA6100 column set and a Model486 variable wavelength UV detector. Prior to analysis, the PLAenantiomeric copolymer is fully hydrolyzed in a basic aqueous solutioninto its constituent D- and L-lactic acid monomer units. Enantiomerratio is conveniently determined by neutralizing the solution with 1NHCl and injecting it into the HPLC through a 0.45 micron filter. Theconcentration of the lactic acid enantiomers is determined by comparisonof the HPLC results to standard curves generated using pure standardssuch as are supplied by the Aldrich and Sigma Chemical Companies.

[0032] The PLA resin may be compounded with a nucleating agent in orderto improve its ability to crystallize quickly. Suitable nucleatingagents include finely divided solids that do not react under theconditions of the heating step or the thermoforming process. Particleshaving a median particle size of less than 5 μm, preferably less than 1μm, are particularly suitable. The most preferred nucleating agent istalc, as it often provides a measure of reinforcement in addition toperforming the nucleation function well. Among the suitable commerciallyavailable talc products are Ultratalc™ 609, available from SpecialtyMinerals, Inc., and Zemex HTP Ultra SC™, available from Zemex FabiBenwood LLC. Nucleating agents are used in effective amounts, but ifused in too large quantities they can cause the physical properties ofthe PLA resin to deteriorate. Preferred amounts for most nucleatingagents are from about 0.1, preferably from about 0.5 to about 10,preferably to about 5, more preferably to about 2.5 percent of thecombined weight of the PLA resin and the nucleating agent. In the caseof talc, it may be desirable to employ larger quantities in order toobtain a desirable reinforcement effect. Thus, preferred usage levelsfor talc are from about 0.5, more preferably from about 3, even morepreferably from about 5 to about 40, more preferably to about 30 andeven more preferably to about 20 percent of the combined weight of thePLA resin and the talc.

[0033] Nucleating agents are conveniently melt compounded into the PLAresin using any suitable melt compounding equipment, such as single andtwin screw extruders, roll mills, Banbury mixers, Farrell continuousmixers and the like. The nucleating agent can also be added during thesheet extrusion process.

[0034] Plasticizers also tend to improve the rate at which PLAcrystallizes. In general, a suitable plasticizer is one that iscompatible with the PLA resin and stable under the conditions of theheating and thermoforming steps. Suitable plasticizers includephthalates (including dioctyl phthalate), citric acid esters, lacticacid esters such as ethyl lactate, lactide esters, mineral oil,triphenyl phosphate, glycerine, acetin and butyrin. Those that arebiodegradable are preferred. Suitable amounts of plasticizer are fromabout 0.5 to about 30 percent, based on the combined weight of the PLAresin plus the plasticizer. Plasticizers are conveniently meltcompounded into the PLA resin or added during the sheet extrusionprocess as described before.

[0035] The ability of a PLA resin to crystallize at a given temperaturecan be expressed in terms of a crystallization half-time, as describedmore fully by Kolstad in “Crystallization Kinetics ofPoly(L-lactide-co-meso-lactide)”, J. Appl. Poly. Sci. 62:1079-1091(1986), incorporated herein by reference. In general, thecrystallization half-time is the time required for a PLA resin toachieve one-half of its ultimate extent of crystallization underparticular heating conditions. Sheets of PLA resins exhibitingcrystallization half-times of less than 10 minutes, preferably less than3 minutes, and especially less than 1 minute at the temperatures of theheating step are suitable. Sheets of PLA resins that exhibitcrystallization half-times of less than about 10 seconds may crystallizetoo rapidly to provide good process control.

[0036] The sheet may be a multilayer type, in which at least one layeris a crystallizable PLA resin as described. Other layers may also be ofcrystallizable PLA resin, an amorphous PLA resin, or may be composed ofa different polymer (such as a barrier plastic), provided that the sheetcan be thermoformed under the conditions described herein. Multilayersheets may be formed by, for example, coextrusion or lamination.

[0037] The sheet may also be cellular. Cellular sheet can be made byincorporating a foaming agent into the PLA resin during the sheetextrusion process, and extruding the sheet under conditions such thatthe foam agent generates a gas and expands the sheet when the sheet isextruded. Extrusion foaming processes are well known and applicablehere. Cellular sheet is preferably mainly closed celled and has adensity as low as about one lb/ft³.

[0038] A cellular sheet can be formed by foaming it during thesoftening/crystallization step of this process. In this case, the sheetwill contain a foaming agent that generates a gas under the conditionsof the heating step. In this manner, the sheet can be simultaneouslyheated and blown. Suitable blowing agents include lower hydrocarbons,halogenated alkanes such as fluorinated alkanes and perfluorinatedalkanes, and water, as well as chemical blowing agents such as citricacid/sodium bicarbonate mixtures, gasses such as carbon dioxide andnitrogen, and the like.

[0039] Formed articles made according to the invention have improvedresistance to heat, compared to PLA articles that are formed with lowercrystallinity. Formed PLA articles having a crystallinity of about 15Joules/gram or more have been found to withstand boiling water forseveral minutes without significant dimensional changes or distortion.As such, the formed articles can be used as containers for hot foods, asmicrowavable food trays, or in other applications where they are exposedto moderately high temperatures (up to about 100° C. in many cases, andup to the crystalline melting point of the article for more highlycrystalline articles). As PLA resin is hydrolyzable, the length of timefor which the article can withstand elevated temperatures will depend onwhether, and how much, water is present. In general, the presence ofliquid water or increasing humidity tends to shorten this time, as thePLA tends to hydrolyze under those conditions.

[0040] The formed articles tend to be opaque to somewhat translucent inappearance, even when a nucleating agent is used. The degree of opacityis generally related to the number and size of the spheruliticcrystallites developed in the crystallization step. The opacity can bereduced through more effective crystal nucleation. More effectnucleation is favored by using smaller nucleating particles, in largernumbers. In some instances, the PLA resin may become oriented during theforming process. This may cause some reduction in opacity in the areasin which the orientation occurs.

[0041] Among the articles that can be made according to the inventionare: beverage cups, other foodware such as trays and plates, includingthose having attached lids; trays and plates for non-food applicationsand separate lids or covers for any of the foregoing.

[0042] The following examples are provided to illustrate the invention,but are not intended to limit its scope. All parts and percentages areby weight unless otherwise indicated.

EXAMPLE 1

[0043] A. Evaluation of Processing Window for Talc-Nucleated PLA sheet

[0044] A single-stage, shuttle-type lab-scale thermoformer without plugassist and a heating oven equipped with an IR radiant heater positionedabove and below the sheet is used to evaluate the processing conditionsfor a nucleated PLA resin sheet. 20-mil sheets made from an amorphous,random copolymer of 98.3% L-lactic acid and 1.7% D-lactic acid andcontaining 10% by weight Ultratalc™ 609 (trademark of SpecialtyMinerals, Inc.) talc are placed for varying, predetermined times in thepre-heated thermoformer oven with the heaters set at 80% power. Afterthe predetermined time in the oven, the sheets are racked out and theirsurface temperature measured using a Raytek™ ST IR surface thermometer.The sheets are then cooled by forcing cool air over their surface. Thecrystallinity of the cooled sheets is measured by DSC according to theprocedure below. In this manner, induced crystallinity is correlatedboth to oven residence time and to the attained surface temperature, asfollows: TABLE 1 Time in Oven (s) Attained Surface Temperature (° C.)Crystallinity (J/g) 16 111 22.1 20 128 30.1 25 143 31.7 30 151 36.4 40162 5.8 55 182 6.5

[0045] The DSC measurements are made using a Mettler Toledo DSC 821ecalorimeter running Star V. 6.0 software. Samples are 5-10 milligrams.Heating is performed from 25-225° C. at 20° C./minute, under air.

[0046] At a heating time of less than 16 seconds under these conditions,the sheet is too rigid to form, even though the measured crystallinitywas less than 22 J/g. At 40-55 seconds residence time, the PLA sheetcontains less than 10 Joules/g crystallinity, indicating that the sheethas reached too high a temperature, thereby destroying thecrystallinity. At more than 55 seconds residence time, the sheet is toofluid to form. Within the range of 16 to 30 seconds heating time, thesheet becomes thermoformable and attains the desired crystallinity.

[0047] Similar evaluations of 20 mil sheets of other PLA resinsdemonstrate how the process window can vary with the composition of thePLA resin. When a PLA resin containing 1.7% D-isomer and 2.5% Ultratalc609 is tested, the optimum oven residence time under these conditions isabout 18-33 seconds. The attained surface temperature that correlates to15 J/g or more crystallinity is about 115-155° C. For a 1.2% D-isomerPLA containing 5% UltraTalc 609, the optimum oven residence time underthese conditions is about 10-23 seconds and the surface temperaturecorrelating to 15 J/g or more crystallinity is 105-155° C.

[0048] B. Thermoforming of PLA Resin Sheet on Cold Mold

[0049] Sheets of amorphous PLA resin as described in A above arethermoformed by heating them in the pre-heated thermoformer oven for 15seconds at 90% power, and then immediately vacuum forming the heatedsheets onto a 23° C., single compartment tray mold having dimensions of3⅛×3⅝×1.5 inches. The time on the mold is varied in order to determinethe minimum cycle time and to assess the effect of in-mold residencetime on crystallinity. All parts come off the mold rigid, regardless ofin-mold residence time. The results are summarized in Table 2 below.TABLE 2 Time on Mold (s) Overall Cycle Time (s)* Crystallinity (J/g) 419 16.7 3 18 16.1 2 17 16.4

[0050] The data in table 2 demonstrate that thermoformed articles ofsemi-crystalline PLA are easily prepared via the process of theinvention. Note that under these conditions, the in-mold residence timehas no effect on crystallinity, indicating that all crystallinity isdeveloped in the thermoformer oven during the heating step. This isconfirmed by DSC evaluation of a sheet that is taken from thethermoformer oven and rapidly cooled without undergoing the formingstep.

[0051] C. Heat Stability of Formed Parts

[0052] The heat resistance of the parts made in B above is evaluated byfilling the part with water and microwaving at high power for fiveminutes, or longer if required to bring the water to boiling. All partsretain their shape under this test. By contrast, a similar part having acrystallinity of less than 10 J/g nearly completely loses its shapeunder this test, flattening almost entirely so that the water itcontains at the start of the test is completely spilled.

EXAMPLE 2

[0053] A 98.8% L/1.2% D PLA resin containing 5% Ultratalc 609 is used inthis Example. It is extruded into 16″ wide, 20 mil sheet using a singlescrew extruder equipped with a general purpose screw, a Maddock mixinghead and a 28″ horizontal cast sheet die. The freshly extruded,substantially amorphous sheet is run through a heated three roll stack,trimmed and wound into rolls.

[0054] This sheet is thermoformed on an Irwin Mini Mag 28 in-linethermoformer equipped with a 6″ soup bowl mold. The machine is operatedat 14″ and 24″ index lengths. Index length affects oven residence time,with larger indices reducing residence time proportionately, for a givencycle time. The oven is operated at a top oven temperature of between480 and 675° F. and a bottom oven temperature of between 430 and 635°F., as indicated in Table 3 below. The temperature of the sheet ismeasured as it leaves the oven just prior to forming, using an IRthermometer. The mold temperature is about 23° C. In-mold residence timeis less than 3 seconds. Samples of the sheet trim and the molded bowlsare taken in order to measure crystallinity. It is found that thecrystallinity of the sheet trim that is not quenched by the mold is notsignificantly different from that of the corresponding bowls. Thisconfirms that crystallization has occurred in the heating step prior toforming. Results are summarized in Table 3. TABLE 3 Index CrystallinityLength Top Oven Bottom Oven Attained Sheet (Molded Parts) (in.) Temp. (°C.) Temp. (° C.) Temp. (° C.) (J/g) 14 480 430 120 16.3 14 490 445 13316.5 14 500 450 ND 21.6 14 505 460 140 25.7 14 510 465 143 26.5 14 520475 143 25.1 14 530 485 149 25.9 24 635 585 137 16.6 24 650 610 146 18.324 675 635 155 14.0

What is claimed is:
 1. A method for making formed heat-resistant PLAarticles, comprising heating a sheet of an amorphous, crystallizable PLAresin until it has obtained a surface temperature from about 80 to about155° C., then thermoforming the heated sheet on a mold that is at atemperature below 80° C.
 2. The method of claim 1, wherein the mold isat a temperature below the T_(g) of the PLA resin.
 3. The method ofclaim 2, wherein the sheet is heated under conditions such that the PLAresin attains a crystallinity of at least 15 Joules/gram.
 4. The methodof claim 3, wherein the PLA resin is a homopolymer of L-lactic acid orD-lactic acid, a random copolymer of L-lactic acid and D-lactic acid, ablock copolymer of L-lactic acid and D-lactic acid or a mixture of twoor more of these.
 5. The method of claim 4 wherein the PLA resincontains a nucleating agent, a plasticizer, or both.
 6. The method ofclaim 5 wherein the PLA resin contains from about 1 to about 40 percentof a finely divided talc, based on the combined weight of the PLA resinand the talc.
 7. The method of claim 4 wherein the PLA resin has acrystallization half-time at the temperature to which it is heatedduring the heating step of less than 3 minutes.
 8. The method of claim 7wherein the sheet is heated under conditions such that the PLA resinattains a crystallinity of at least 20 Joules/gram.
 9. The method ofclaim 8 wherein the sheet is heated under conditions such that the PLAresin attains a crystallinity of at least 24 Joules/gram.
 10. The methodof claim 1 wherein the PLA sheet contains one or more additives selectedfrom the group consisting of nucleants, other inorganic fillers,plasticizers, reinforcing agents, slip agents, lubricants,UV-stabilizers, thermal stabilizers, flame retardants, foaming agents,antistatic agents, antioxidants and colorants.
 11. The method of claim 1wherein the amorphous PLA sheet is cellular.
 12. The method of claim 1wherein the amorphous PLA sheet contains a foaming agent, and expandsduring the heating step to become cellular.
 13. The method of claim 1wherein the PLA resin contains a plasticizer.
 14. The method of claim 1wherein the sheet of amorphous PLA resin forms a layer of a multilayerstructure having at least one other layer which is not a crystallizablePLA resin.
 15. A thermoformed article made according to the process ofclaim
 1. 16. A method for making formed heat-resistant PLA articles,comprising heating a sheet of a semicrystalline PLA resin having acrystallinity of at least 10 Joules/gram to a temperature at which thesheet can be thermoformed but at which the crystallinity of the sheet isnot reduced below 10 Joules/gram, and then thermoforming the sheet on amold that is at a temperature below 80° C. to produce a formed articleof PLA resin having a crystallinity of at least 10 Joules/gram.
 17. Amethod for making formed heat-resistant PLA articles, comprising heatinga sheet of an amorphous, crystallizable PLA resin to a temperature atwhich the PLA resin forms crystallites and for a time sufficient toimpart a crystallinity of at least 10 Joules/gram to the PLA resin, andthen thermoforming the heated sheet on a mold that is at a temperaturebelow 80° C. to produce a formed article of PLA resin with acrystalinity of at least 10 Joules/gram.